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Buy too little finish and you run out between coats, which on a tinted stain can mean a visible change where the new can starts. This works out how much stain, oil, polyurethane or paint a surface needs, from its size, the number of coats and the finish you are using, so you pick up the right amount in one trip.
Area times coats, divided by reach
Finish coverage is simple arithmetic: the area you are coating, multiplied by the number of coats, divided by how far the finish spreads. A quart or a gallon lists a spread rate on the can, and that tells you the reach. Three coats over a six square foot tabletop is eighteen square feet of coating, which barely dents a gallon of poly. Most small projects need far less finish than people buy, which is why a modest can lasts across several jobs.
The first coat drinks the most
Bare wood, especially open-grained species like oak or ash, soaks up the first coat and gives back less coverage than the can promises. Later coats lie on a sealed surface and spread much further. So the spread rates here are typical averages; expect the first coat to cover less and the finish to go further after that. Thinning the first coat to a sealer or wiping it on helps it reach, and evens out how much the wood takes.
Count every face you finish
This figures one face. A tabletop finished top and bottom, as it should be to stay flat, is two faces plus the edges, so measure all of it or just double the one-face figure and add a little. A box or a cabinet has inside and outside. It is easy to work out the pretty top and forget the four edges and the underside, then come up short. When in doubt, measure generously.
Buy a margin
Finish is cheap next to the wood and the hours, and running dry mid-project costs you both time and, on colored finishes, a possible mismatch between cans. So round up and keep a little spare, which also covers touch-ups later. Leftover clear finish stores for a good while if the can is sealed and kept full; decant it into a smaller jar to cut the air, and it will be ready for the next piece.
More MakerKiln tools
- board feetBoard Foot CalculatorBoard feet and cost for a whole lumber list, with a waste allowance.→
- epoxy coverageEpoxy Coverage CalculatorResin for a flood coat or deep pour, from the area and depth.→
- wood movementWood Movement CalculatorSeasonal expansion and shrinkage by species, width, grain and moisture swing.→
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